Sylvanas will get “redeemed” and become a live high elf again. The horde will then have “high elves” (honestly just blue eyed blood elves) join their faction because the remaining high elves will see Sylvanas as a hero that actually died fighting for Quel’thalas unlike her traitorous sisters.
Nathanos will be killed, either by Greymane or Tyrande, or as a part of the effort to redeem Sylvanas.
And as for Lilian Voss? In this expansion there is dialogue to show they’ve made her pro horde/Forsaken, because spending 5-6 years as the very thing you hate will change your perspective on things. Lilian will eventually replace Sylvanas as the Forsaken faction leader.
Sylvanas will step down as warchief, and Saurfang will step up. She’ll move back to Quel’thalas and be active in politics with Lor’themar, Liadrin, Rommath, and Halduron.